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The Effects of Temperature and CO2 Trends on the 1981-1999 Greening of North America and Eurasia
Volume 5, Number 38: 18 September 2002

In a paper entitled "Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999," Zhou et al. (2001) determined that the magnitude of the satellite-derived normalized difference vegetation index (NDVI) rose by 8.44% and 12.41%, respectively, in North America and Eurasia over this period. Noting that the NDVI "can be used to proxy the vegetation's responses to climate changes because it is well correlated with the fraction of photosynthetically active radiation absorbed by plant canopies and thus leaf area, leaf biomass, and potential photosynthesis," the scientists went on to suggest that the increases in plant growth and vitality implied by their NDVI data were primarily driven by concurrent increases in near-surface air temperature, which were said by them to have been significant throughout most of Eurasia but much smaller in North America, especially the United States, where temperatures may have actually declined throughout the eastern part of the country over the period of their study.

After lying dormant for about a year, Zhou et al.'s attribution of this "greening" phenomenon to concurrent increases in near-surface air temperature was recently challenged by Ahlbeck (2002), who suggests that the observed upward trends in NDVI were primarily driven by the concurrent increase in the air's CO2 content, and that fluctuations in temperature were primarily responsible for variations about the more steady upward trend defined by the increase in CO2. In replying to this challenge, Kaufmann et al. (2002) claim Ahlbeck is wrong and reaffirm their initial take on the issue. So who is correct? Or should we say who is more correct?

In their reply to the challenge raised by Ahlbeck, Kaufmann et al. attempt to discredit his analysis by saying the reason the NDVI data are better described by temperature and CO2 variations together than by temperature variations alone - which fact they do not deny - is that the CO2 variation over the period in question is tightly coupled to the time variable, which does an equally good - if not marginally better - job of explaining the NDVI trends. In an Internet discussion of the issue, however, Ahlbeck replies that it is wrong to use time in this way; for there is no conceivable reason for the passage of time alone to stimulate plant growth from one year to the next, while there is a very good reason for expecting the increase in atmospheric CO2 concentration to do so: it has been demonstrated to do precisely that in literally thousands of plant growth experiments.

Kaufmann et al. likely saw this criticism coming when they wrote their reply to Ahlbeck; for they ended their discussion of the issue by acknowledging that time was just a stand-in for something else. Nevertheless, all they could say about the phantom parameter was that "the mechanism that lays behind the linear increase in NDVI is uncertain but could include forest regrowth following the effects of human disturbance and/or the decay of the increase in aerosol optical depth associated with the volcanic eruption of El Chichon at the start of the sample period." By making this statement, however, they essentially admitted that temperature was likely not the predominant parameter responsible for the observed increase in NDVI, in contradiction of what was originally suggested by Zhou et al.

At this point, the path that leads to truth should be crystal clear; we need to quantitatively evaluate the likely effects of the various mechanisms that have been proposed to explain the mean trends of the North American and Eurasian NDVI measurements from 1981 to 1999. And since we have little desire to waste our time on the "uncertain" mechanisms proposed by Kaufmann et al., we will begin - and end! (because it works so well) - with an analysis of the aerial fertilization effect of atmospheric CO2 enrichment, as suggested by Ahlbeck.

Since "forests and woodlands" are the words used by Zhou et al. to describe the areas they surveyed, we must first ask ourselves how much a specified increment in the air's CO2 concentration increases tree growth. In a massive review of the pertinent literature, Saxe et al. (1998) concluded that "close to a doubling" of the ambient atmospheric CO2 concentration leads to an approximate 50% increase in the biomass production of angiosperm trees and a 130% increase in the biomass production of coniferous species, where "close to a doubling" of CO2 is probably somewhere in the range of 330 to 360 ppm. Averaging both sets of numbers to get a single approximation of the biomass-enhancing effect of atmospheric CO2 enrichment on a reasonable mix of all types of trees, we get a mean growth stimulation factor of 78% for a 300 ppm increase in the air's CO2 concentration.

This stimulation factor is essentially identical to the long-term equilibrium growth response derived by Idso and Kimball (2001) for evergreen sour orange trees in what has become the longest continuous atmospheric CO2 enrichment study ever to be conducted (13 years and counting), i.e., an 80% increase in biomass in response to a 300 ppm increase in atmospheric CO2. More recently, Leavitt et al. (2002) determined that the CO2-induced enhancement in the sour orange trees' growth is identical to the CO2-induced enhancement in the sour orange trees' water use efficiency, as derived from totally independent stable-carbon isotope measurements of the trees' trunks and leaves.

This exact correspondence likely results from the fact that atmospheric CO2 enrichment has little to no effect on leaf stomatal conductances in sour orange trees, as observed by Idso et al. (1993), so that nearly all of the CO2-induced increase in water use efficiency is caused by the CO2-induced increase in biomass. This latter observation resonates with the results of the literature review of Saxe et al., who report that "increasing numbers of experiments show a lack of stomatal sensitivity to CO2," especially when the data come from "long-term experiments on larger trees rooted directly in the ground," as in the case of Idso and Kimball's study and as has also been found to be true by Eamus (1996).

Our reason for reporting this close correspondence between CO2-induced increases in woody-plant biomass and water use efficiency is that it allows us to make reasonable comparisons between experimentally-derived and field-observed growth responses of trees by means of comparisons between experimentally-derived and field-observed water use efficiency responses of trees. Feng (1999), for example, assembled and analyzed a large array of stable-carbon isotope data for 23 different sets of trees scattered throughout western North America. For the period 1800 to 1985, over which time the air's CO2 concentration rose by approximately 62 ppm, the intrinsic water use efficiencies of the 23 groups of trees rose from 10 to 25%, or by a mean of 17.5%, which is equivalent to an increase of approximately 85% per 300 ppm increase in the air's CO2 concentration. In addition, even larger field responses have been reported by Bert et al. (1997), who measured CO2-induced water use efficiency increases of 150% per 300 ppm increase in atmospheric CO2 between 1860 and 1980 in white fir in France, while Hemming (1998) observed similar increases between 1895 and 1994 in European beech, oak and pine trees.

In light of these several real-world observations, therefore, and simply by applying the middle-of-the-road results of Idso and Kimball (2001) - an 80% increase in tree growth for a 300 ppm increase in atmospheric CO2 - we calculate that the 28.6-ppm increase in the air's CO2 concentration that occurred between 1981 and 1999 should have increased mean tree growth around the world by something on the order of 7.6%. This result is especially gratifying, inasmuch as Eurasia (which supposedly experienced a large concomitant warming) exhibited a 12.4% increase in NDVI, while North America (which warmed far less and may have even cooled in some areas) exhibited a much reduced 8.4% increase in NDVI, which is only slightly greater than what would be predicted on the basis of the increase in CO2 alone.

These observations, together with those of Zhou et al. and Ahlbeck, clearly indicate that warming does indeed lead to increases in woody-plant biomass production, as originally suggested by Zhou et al. and recently reaffirmed by Kaufmann et al. However, they also demonstrate that the growth stimulation provided by the temperature increase of the last two decades of the 20th century was largely overshadowed by the much stronger growth stimulation provided by the concomitant rise in the air's CO2 content. In the end, therefore, it is Ahlbeck who is clearly the "more correct" of the two camps that have recently debated this issue. Indeed, he is actually totally correct.

Looking at the results from another perspective, it is easy to see that the biological consequences of the increase in the air's CO2 content over the last two decades of the 20th century were strongly positive, and that the consequences of the "unprecedented" concomitant warming experienced throughout Eurasia only gave added value to the benefits provided by the increase in CO2. Which leads us to ask ... Is this the stuff of which catastrophes are made?

Sherwood, Keith and Craig Idso

References
Ahlbeck, J.R. 2002. Comment on "Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999" by L. Zhou et al. Journal of Geophysical Research 107: 10.1029/2001389.

Bert, D., Leavitt, S.W., Dupouey, J.-L. 1997. Variations in wood 13C and water-use efficiency of Abies alba during the last century. Ecology 78: 1588-1595.

Eamus, D. 1996. Responses of field grown trees to CO2 enrichment. Commonwealth Forestry Review 75: 39-47.

Feng, X. 1999. Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: A response to atmospheric CO2 concentration. Geochimica et Cosmochimica Acta 63: 1891-1903.

Hemming, D.L. 1998. Stable Isotopes in Tree Rings: Biosensors of Climate and Atmospheric Carbon-Dioxide Variations. Ph.D. Dissertation. University of Cambridge, Cambridge, UK, 270 p.

Idso, S.B. and Kimball, B.A. 2001. CO2 enrichment of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153.

Idso, S.B., Kimball, B.A., Akin, D.E. and Kridler, J. 1993. A general relationship between CO2-induced reductions in stomatal conductance and concomitant increases in foliage temperature. Environmental and Experimental Botany 33: 443-446.

Kaufmann, R.K., Zhou, L., Tucker, C.J., Slayback, D., Shabanov, N.V. and Myneni, R.B. 2002. Reply to Comment on "Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999: by J.R. Ahlbeck. Journal of Geophysical Research 107: 10.1029/1001JD001516.

Leavitt, S.W., Idso, S.B., Kimball, B.A., Burns, J.M., Sinha, A. and Stott, L. 2002. The effect of long-term atmospheric CO2 enrichment on the intrinsic water-use efficiency of sour orange trees. Chemosphere - Global Change Science, in press.

Saxe, H., Ellsworth, D.S. and Heath, J. 1998. Tree and forest functioning in an enriched CO2 atmosphere. New Phytologist 139: 395-436.

Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback, D., Shabanov, N.V. and Myneni, R.B. 2001. Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999. Journal of Geophysical Research 106: 20,069-20,083.